Will a water-damaged passport affect visa extension or exit from Thailand?

Based on my experience with a water damaged passport yes. He best get a new one. I was nearly denied entry into one country and detained while superiors talked with the passport control. They told me to get a new passport before coming back. Then I was questioned a lot on my return into the U. S. Not fun experiences. Only blank pages were water damaged.

As stated, the first thing to do is get a new passport.

I would err on the safe side and get a new passport...it’s actually up to us to ensure our passport remains undamaged, or to replace it.

It will be very inconvenient, and costly, should entry to any country be denied and then be stuck in limbo until a replacement passport is obtained.

No one here can say it’s ok or not, only the immigration officer on arrival ... or your embassy, but I can tell you now they will also advise you to replace a damaged passport (as they can’t say whether an immigration officer of another country will accept it...it’s completely up to the discretion of the immigration officer)

If it’s IC passport, better to go to your embassy to check. maybe it need to be renewed.. I have a friend who got his IC passport (jus one year old) wet and had to renew.

If the stamps are illegible it's time for a new one.

As we can not see how much damage the passport has from our computer, I would say he is okay to travel, till an Immigration Officer decide that the passport is to damaged and he need a new passport before continue his travel. Best option: Visit an Immigrattion Office or his Embassy to check the passport.